(1) Field of the Invention
The present invention relates to a process for the preparation of a printed draw-formed body. More particularly, the present invention relates to an improvement in the process for preparing a cup-shaped container or a cup from a sheet-like blank such as a metal sheet by draw-forming, wherein the portion of the sheet-like blank, which corresponds to the side face of a final draw-formed body, is printed in advance to form a printed image having no error in the state of the draw-formed body. More particularly, the present invention relates to a process for the preparation of a printed draw-formed body in which formation of a printing plate from a printing original is performed by a computer image processing. Moreover, the present invention relates to a printed draw-formed can on which a printed image excellent in the linearily, which has no error, is formed while taking anisotropy of the plastic flow of a metal blank into consideration, and a process for the preparation of this can.
(2) Description of the Prior Art
Draw-forming of a surface-treated steel sheet such as tinplate or tin-free steel sheet or a metal sheet such as an aluminum sheet has been broadly utilized for the production of a so-called seamless can (two-piece can) or a metal cup. Coating or printing of respective draw-formed bodies after the draw-forming operation is troublesome and requires complicated operations. Accordingly, it is preferred that a sheet-like blank be coated or printed before draw-forming.
In draw-forming, the plastic flow is caused in a sheet-like blank to be formed into a cup-shaped body. In the portion of the sheet-like blank, which is to be formed into a peripheral side wall of the final draw-formed body, the plastic flow comprises not only elongation in the direction of the height of the formed body but also contraction in the radial direction.
In the prior printing of a sheet-like blank for draw-forming, in view of the above-mentioned plastic flow of the blank, it is necessary that an original which is rectangular in the developed state should be converted to an annular printing plate, and a handwriting method or optical conversion method has been heretofore adopted for this conversion.
According to the handwriting method, a rectangular transparent original having ruled squares is wound around the peripheral side of a draw-formed body, and an annular developed plane having squares corresponding to the ruled squares of the original is painted by handwriting in the area corresponding to the image area of the original to form a printing plate for a sheet-like blank. According to this method, a high skill is necessary for the preparation of the printing plate, and furthermore, great costs and long time are required. Moreover, the printed image actually reproduced on the peripheral side wall of the draw-formed body is still insufficient in the precision and the like.
A technique of preparing a printing plate by optically developing a rectangular original annularly has already been proposed. For example, Japanese Patent Publication No. 11388/70 discloses a process in which a rectangular image is photographed in such a manner that the size is shortened in the direction of the height, the shortened photo is wound around a mandle and the photo is photographed again through a frustoconical mirror arranged around the mandrel to form a printing plate having an annularly developed image. Moreover, Japanese Patent Publication No. 23455/73 discloses a process in which a frustoconical prism having a hole having an inner diameter which is substantially equal to the outer diameter of a can is arranged at the center, a rectangular original is located in the inner surface of the hole, and the original image is photographed by a camera through the above-mentioned prism and a compensating lens to form a printing plate having an annular plane.
However, these methods are defective in that troublesome operations and particular optical devices are necessary, it is difficult to reproduce a fine dot pattern at a high precision because of aberration of the optical system, and every time when the original of the draw-formed body is changed, the troublesome conversion operation has to be carried out to form a new printing plate.
In each of metal blanks for cans, the thickness is reduced by a high-degree rolling operation, and because of the influence of this rolling, anisotropy is caused in the plastic flow of the blank at the draw-forming step. Accordingly, the printed image is readily disturbed in the side wall of the obtained draw-formed can, especially the upper portion of the side wall, and the precision or linearity of the image is often degraded.